Low-Voltage Organic Thin Film Transistors with High-K Bi1.5Zn1.0nb1.5O7 Pyrochlore Gate Insulator

Thin film transistor circuits using organic semiconductors (oTFT) have received intense interest for applications requiring structural flexibility, large area coverage, low temperature processing, and low-cost [1]. Pentacene TFTs have demonstrated the highest performance among TFTs with an organic semiconductor channel. A major limitation, however, has been unusually high operating voltages (20~100 V), a concern for portable, battery-powered device applications [2]. The high-operating voltage stems from poor capacitive coupling between the gate electrode and channel region. A combination of higher permittivity gate dielectric and reduced dielectric thickness leads to lower voltage operation. Recently, M. Hallk et al. reported low-voltage pentacene OTFTs with very thin (2.5 nm) amorphous molecular gate dielectric [3]. Flexible polymer substrates, characterized by rough surfaces, benefit from the use of high K dielectrics given the ability to accommodate thicker films without the need to increase operating voltage, which leads to the suppression of pinholes and minimizes problems associated with step coverage. We successfully fabricated low voltage (< 3V) organic transistors using a 200 nm thick pyrochlore gate dielectric, Bi1.5Zn1.0Nb1.5O7 (BZN), with the highest reported dielectric constant (3r =50) prepared at a room temperature. The introduction of an extremely thin parylene film between the BZN dielectric and the pentacene semiconductor markedly shifted the threshold voltage, making it possible to fabricate both enhancement (E) and depletion (D) TFTs. Positive threshold voltage and the threshold voltage change caused by parylene may be due to dangling bonds at the BZN surface and the passivation of the dangling bonds. The inverters with depletion load were operated at less than 4V and had an excellent noise margin. The inverter and its performance were the best among the inverters made with OTFTs, with similar gate dielectric thicknesses reported previously in the literature.